As conventional electronic watches, there are electronic watches provided with built-in electric power generator for converting external energy such as photovoltaic energy, mechanical energy, or the like into electric energy so that the electric energy can be utilized as driving energy for executing time display operation.
Among such electronic watches provided with the built-in electric power generator, there are included a solar cell watch using a solar cell, a mechanical electric power generation watch for utilizing electric energy converted from mechanical energy generated by rotation of a rotary weight, and a temperature difference electric power generation watch for generating electric power by utilizing the difference in temperature between the opposite ends of each of integrated thermocouples.
It is essential for these electronic watches provided with the built-in electric power generator to have built-in means for storing external energy therein while it is available so that the watch is driven continuously and stably all the time even after the external energy is gone.
For this reason, an electronic watch with a charging function, incorporating means for storing external energy therein, has been disclosed in, for example, Japanese Patent Laid-open Publication H 6-31725. An outline of circuits in the vicinity of a power supply source of the electronic watch is described with reference to FIG. 13.
An electric power generator 10, which is a solar cell, a first diode 11 and a capacitor 23 of small capacitance form a closed circuit, and further, a clocking block 24 for executing time display operation by use of electric energy and the capacitor 23 are connected in parallel. The electric power generator 10, a second diode 12, first switch 13, and secondary power source 31 form another closed circuit. A second switch 14 interconnects the positive source of the capacitor 23 and that of the secondary power source 31 in such a way as to connect the capacitor 23 and the secondary power source 31 in parallel.
Further, a first voltage comparator 16 controls the first switch 13 by comparing the terminal voltage of the capacitor 23 with a predetermined threshold value, and a second voltage comparator 17 controls the second switch 14 by comparing the terminal voltage of the secondary power source 31 with that of the capacitor 23.
In this electronic watch, as soon as electric energy is generated by the electric power generator 10, the capacitor 23 is immediately charged with electric energy, and the clocking block 24 is actuated by the electric energy stored in the capacitor 23.
When the terminal voltage of the capacitor 23 reaches a predetermined level or a level higher than that, the first switch 13 is closed by the agency of the first voltage comparator 16, and the secondary power source 31 is charged with the electric energy generated by the electric power generator 10.
When electric energy is not generated by the electric power generator 10, the terminal voltage of the capacitor 23 declines due to consumption of the electric energy by the clocking block 24, but when the terminal voltage of the secondary power source 31 is compared with that of the capacitor 23 by the second voltage comparator 17, and found to be higher than the latter, the second switch 14 is closed, thereby enabling continued operation of the clocking block 24 by the electric energy stored in the secondary power source 31.
The terminal voltage of the secondary power source 31, however, varies depending on the amount of electric energy stored, and with reference to a generated voltage of the electric power generator 10, there will arise a problem of the generated voltage undergoing changes depending on an external environment in the case of such an electric power generation device as represented by a thermoelectric device although there will arise no problem with a constant-voltage power-generation device such as a solar cell generating a substantially constant voltage all the time.
For example, in the circuit diagram in FIG. 13, wherein electric energy is generated by the electric power generator 10, if there exists a relationship of
(terminal voltage of the secondary power source 31) PA1 &lt;(terminal voltage of the capacitor 23) PA1 &lt;(threshold value of the first voltage comparator 16)
and the generated voltage of the electric power generator 10 is higher than that of the secondary power source 31, the second switch 14 as well as the first switch 13 are controlled to be turned off even though the electric power generator 10 is capable of charging the secondary power source 31. As a result, the secondary power source 31 will not be charged with the result that effective use of the electric energy generated can not be made.
Accordingly, when the terminal voltage of the secondary power source 31 is relatively low, and the generated voltage is not so high, a charging operation is not executed, resulting in poor charging efficiency.
This is because the decision on whether or not the electronic watch is in a condition to be able to charge the secondary power source 31 is made only on the basis of the threshold value of the first voltage comparator 16.
It is therefore an object of the invention to solve the problem described above so that the charging operation of the electric energy storage means can be executed efficiently even if the terminal voltage of the electric power generator or the electric power storage means undergoes changes.